Isotelus mafritzi scale in cm   CLASS - Trilobita ORDER - Ptychopariida FAMILY - Asaphidae GENUS/SPECIES - Isotelus mafritzi ROCK UNIT - Cobourg Formation AGE - Middle Ordovician (465 million years old) LOCALITY - Bowmanville, Ontario, Canada The very first trilobites had spiny tails in which most or all of the tail segments were individually covered in outer shell material. Isotelus represents one of the later trends in trilobite evolution; simplifying the outer shell or "exoskeleton". Many later trilobites, including Isotelus, had an outer shell with the tail segments combined into one large, smooth tail shield or "pygidium". To learn more about Isotelus. PRI 42104